As Ultra Wideband (UWB) communication becomes increasingly desirable for wireless devices due to its speed and capacity combined with its resilience to interference within high-frequency bands, design of certain RF modules becomes a great challenge due to the unique characteristics of the UWB signal environment. In particular, the Variable Gain Amplifier (VGA) is a common and important RF module whose design poses unique challenges in the UWB environment.
In RF interfaces associated with UWB receivers, VGA stages must meet high demands of linearity within the characteristic bandwidth which can lead to problems especially in the case of input signals having a wide dynamic range or in the case of UWB systems, a wide transmission bandwidth. Since gain and bandwidth are inversely proportional in VGAs, the greater the bandwidth, the less gain can typically be realized.
The challenges of RF signal recovery, processing, and gain adjustment in UWB systems is that, from a design standpoint, UWB signals or chips are transmitted in an environment which may have widely different characteristics between any one transmitter and any one or more receivers. Thus the input amplifier of a typical receiver must process UWB signals received within a correspondingly wide amplitude range. As noted, the VGA in a UWB receiver must possess characteristics of good linearity, optimum offset, and acceptable noise rejecting performance. The UWB signal must further be passed without significant loading or interference introduced by the amplifier through factors such as impedance mismatches and the like.
Conventional high frequency receivers typically further use one or more Intermediate Frequency (IF) down-converter stages. IF down-conversion stages lower the frequency of, or “down-convert” information signals transmitted across, for example, an air interface from transmission band frequencies, for example in the GHz band, to intermediate information signals at frequencies in lower frequency bands, for example the MHz band. In some conventional receivers, many IF stages are needed or are used with the effect of complicating design, increasing power consumption and producing additional disadvantageous consequences.
As also noted, for a conventional receiver with many IF stages between the actual RF carrier frequency and the frequency of the baseband stage, problems arise in that each IF stage or filter adds costs in the form of additional expense, additional power, additional complexity, additional circuit board real-estate, and the like to the receiver design and can degrade performance. It will be appreciated that much of the cost associated with additional IF stages can originate from the additional support circuitry associated therewith. For example, each down-conversion involves the addition of a separate local oscillator (LO) source usually including a separate phase-locked loop (pLL) and associated discrete components associated therewith. Additional circuits may also be required for input and output impedance matching.
IF conversion oriented receivers or RF stages typically suffer from serious drawbacks in the UWB environment associated with the inability to adequately control parameters such as DC offset which can arise from impedance mismatches, and the like, and can present difficulties in UWB systems by masking signal information, particularly since signal levels in UWB systems are deliberately low. Further problems arise with the use of IF stages in UWB systems in that RF interference associated with residual signals generated by the LO circuits associated with each IF stage, including the LO signals themselves, can be introduced into the signal path and can be amplified along with the signal or can prevent accurate signal recovery.
One solution to the problems associated with use of IF conversion in UWB systems includes using direct conversion receivers. Direct conversion receivers perform high speed conversion of the input signals “directly” from transmission band or a band close to the transmission band. However, even with the use of direct conversion techniques, DC offset can still be a problem particularly when passed through the signal path to high speed analog to digital converters. In addition to DC offset, gain setting must be accomplished with speed and precision prior to conversion further adding to the challenges of signal recovery. Thus, it will be appreciated that an RF processing circuit is needed for removing DC offset and for performing gain adjustment in UWB receivers.